The Rebbe recounts that after Yom Kippur, his father emphasized the need for teshuvah, especially with joy. Even though teshuvah can bring bitterness, true reflection and trust in G-d's kindness lead to great happiness. All Divine service, including teshuvah, must be joyful.
https://www.torahrecordings.com/rebbe/igroskodesh/005/001_tishrei/1226